1. Field of the Invention
The present invention generally relates to a charging/discharging apparatus, and particularly to a battery charging/discharging apparatus and a battery charging/discharging method in which the remaining capacity of a battery can be determined with high accuracy.
2. Description of the Related Art
FIG. 1 is a block diagram showing an example structure of a conventional battery pack 1.
The battery pack 1 includes cells 12-1 and 12-2 connected in series (hereinafter collectively referred to as “battery 12”, unless the cells 12-1 and 12-2 should be discriminated), which may comprise lithium ion batteries, for powering electronic devices. The positive electrode of the cell 12-1 is connected to a battery terminal 11-1 while the negative electrode of the cell 12-2 is connected to a battery terminal 11-2 via a resistor 13 provided for current detection and a protection device 14. When the battery pack 1 is attached to an electronic device (not shown), the battery terminals 11-1 and 11-2 are connected to connection terminals of the electronic device so that the electronic device can be powered by the battery 12.
A cell voltage detector 15 detects a cell voltage when the cell 12-1 and the cell 12-2 are connected in series, and outputs the detection result to a microcomputer 17. A charging/discharging current detector 16 detects the direction and value of voltage drop due to the current flowing to the resistor 13 in order to detect the existence of a charging current or a discharging current. The result is then output to the microcomputer 17. The protection device 14 protects the battery 12 from being overcharged or overdischarged.
The microcomputer 17 calculates the remaining battery capacity value based on the cell voltage supplied from the cell voltage detector 15, and stores the resulting value in an internal memory 17a. The microcomputer 17 switches to wake-up mode if a charging current or a discharging current is supplied from the charging/discharging current detector 16 or if it receives incoming data from the outside. If no charging/discharging current is supplied within a predetermined time or if no incoming data is received while it is in wake-up mode, the microcomputer 17 switches to sleep mode.
FIG. 2 is a flowchart of the process for calculating the remaining battery capacity value of the battery pack 1 shown in FIG. 1.
At step S1, the microcomputer 17 determines whether or not the battery 12 has been charged, i.e., whether or not a charging current has been supplied from the charging/discharging current detector 16. If it is determined at step S1 that the battery 12 has been charged, the process proceeds to step S2, in which the additional capacity value is calculated by equation (1) as follows:additional capacity value=charging current value×sampling interval  (1)
After the additional capacity value has been calculated at step S2, at step S3 the microcomputer 17 reads the remaining capacity value stored in the memory 17a, and calculates the up-to-date remaining capacity value by equation (2) as follows:up-to-date remaining capacity value=remaining capacity value read from the memory 17a+additional capacity value  (2)
At step S3, the microcomputer 17 stores the up-to-date remaining capacity value calculated by equation (2) in the memory 17a or updates the existing remaining capacity value, and the process returns to step S1 to iterate the above-described operation.
If it is determined at step S1 that the battery 12 has not been charged, the process proceeds to step S4, in which the microcomputer 17 further determines whether or not the battery 12 has been discharged, i.e., whether or not a discharging current has been supplied from the charging/discharging current detector 16. If it is determined at step S4 that it has not been discharged, the process returns to step S1 to iterate the above-described operation.
If it is determined at step S4 that the battery 12 has been discharged, the process proceeds to step S5, in which the microcomputer 17 calculates the reduced capacity value by equation (3) as follows:reduced capacity value=discharging current value×sampling interval  (3)
After the reduced capacity value has been calculated at step S5, at step S6, the microcomputer 17 reads the remaining capacity value stored in the memory 17a, and calculates the up-to-date remaining capacity value by equation (4) as follows:up-to-date remaining capacity value=remaining capacity value read from the memory 17a−reduced capacity value  (4)
At step S6, the microcomputer 17 stores the up-to-date remaining capacity value calculated by equation (4) in the memory 17a or updates the existing remaining capacity value, and the process returns to step S1 to iterate the above-described operation.
Accordingly, the calculation process using a charging current or a discharging current is performed to determine the remaining battery capacity.
The microcomputer 17 utilizes the power supplied from the battery 12 in order to determine the remaining battery capacity. Hence, when the battery is in standby mode (when the remaining battery capacity calculation process is not performed), the microcomputer 17 switches to sleep mode in order to reduce the power consumption of the microcomputer 17. This may suppress discharge current (leakage current) of the battery 12 in standby mode.
When the charging/discharging current detector 16 detects a charging current or a discharging current, the microcomputer 17 switches to wake-up mode; however, discharge current (leakage current) when the battery is in standby mode is minute, and thus cannot be detected by the charging/discharging current detector 16.
As a result, when the battery pack 1 is in standby mode, a trace amount of discharge current (leakage current) flows, but the accumulated current (standby current×standby time) cannot be measured or detected because the microcomputer 17 is in sleep mode. Therefore, if the battery pack 1 is in standby mode for a long time, there is a problem in that the remaining capacity value stored in the memory 17a by the microcomputer 17 might be relatively greater than the actual remaining capacity value (actual capacity value).
If the microcomputer 17 is forced not to enter sleep mode and is allowed to detect the standby current, the current will be minute, and will not be able to be precisely detected. There still exists such a problem that an error occurs between the remaining capacity value stored in the memory 17a and the actual capacity value.